A Screen for Membrane Fission Catalysts Identifies the ATPase EHD1

Reconstituting membrane fission using a high content and throughput assay

Protein-mediated membrane fission has been analyzed both in bulk and at the single event resolution. Studies on membrane fission in vitro using tethers have provided fundamental insights into the process but are low in throughput. In recent years, supported membrane template (SMrT) have emerged as a facile and convenient assay system for membrane fission. SMrTs provide useful information on intermediates in the pathway to fission and are therefore high in content. They are also high in throughput because numerous fission events can be monitored in a single experiment. This review discusses the utility of SMrTs in providing insights into fission pathways and its adaptation to annotate membrane fission functions in proteins.

Advances and challenges in understanding endosomal sorting and fission

Endosomes play crucial roles in the cell, serving as focal and 'triage' points for internalized lipids and receptors. As such, endosomes are a critical branching point that determines whether receptors are sorted for degradation or recycling. This Viewpoint aims to highlight recent advances in endosome research, including key endosomal functions such as sorting and fission. Moreover, the Viewpoint addresses key technical and conceptual challenges in studying endosomes.

Toshima J, Toshima J. Role of phosphatidylserine in the localization of cell surface membrane proteins in yeast

Phosphatidylserine (PS) is a constituent of the cell membrane, being especially abundant in the cytoplasmic leaflet, and plays important roles in a number of cellular functions, including the formation of cell polarity and intracellular vesicle transport. Several studies in mammalian cells have suggested the role of PS in retrograde membrane traffic through endosomes, but in yeast, where PS is localized primarily at the plasma membrane (PM), the role in intracellular organelles remains unclear. Additionally, it is reported that polarized endocytic site formation is defective in PS-depleted yeast cells, but the role in the endocytic machinery has not been well understood. In this study, to clarify the role of PS in the endocytic pathway, we analyzed the effect of PS depletion on endocytic internalization and post-endocytic transport. We demonstrated that in cell lacking the PS synthase Cho1p (cho1Δ cell), binding and internalization of mating pheromone α-factor into the cell was severely impaired. Interestingly, the processes of endocytosis were mostly unaffected, but protein transport from the trans-Golgi network (TGN) to the PM was defective and localization of cell surface proteins was severely impaired in cho1Δ cells. We also showed that PS accumulated in intracellular compartments in cells lacking Rcy1p and Vps52p, both of which are implicated in endosome-to-PM transport via the TGN, and that the number of Snx4p-residing endosomes was increased in cho1Δ cells. These results suggest that PS plays a crucial role in the transport and localization of cell surface membrane proteins.Key words: phosphatidylserine, endocytosis, recycling, vesicle transport.

Coronin2A links actin-based endosomal processes to the EHD1 fission machinery

Fission of transport vesicles from endosomes is a crucial step in the recycling of lipids and receptors to the plasma membrane, but this process remains poorly understood. Although key components of the fission machinery, including the actin cytoskeleton and the ATPase Eps15 homology domain protein 1 (EHD1), have been implicated in endosomal fission, how this process is coordinately regulated is not known. We have identified the actin regulatory protein Coronin2A (CORO2A) as a novel EHD1 interaction partner. CORO2A localizes to stress fibers and actin microfilaments but also can be observed in partial overlap with EHD1 on endosomal structures. siRNA knockdown of CORO2A led to enlarged lamellae-like actin-rich protrusions, consistent with a role of other Coronin family proteins in attenuating actin-branching. Moreover, CORO2A depletion also caused a marked decrease in the internalization of clathrin-dependent cargo but had little impact on the uptake of clathrin-independent cargo, highlighting key differences in the role of branched actin for different modes of endocytosis. However, CORO2A was required for recycling of clathrin-independent cargo, and its depletion led to enlarged endosomes, supporting a role for CORO2A in the fission of endosomal vesicles. Our data support a novel role for CORO2A in coordinating endosomal fission and recycling with EHD1. [Media: see text].

Metal-Binding Propensity in the Mitochondrial Dynamin-Related Protein 1

Dynamin-related protein1 (Drp1) functions to divide mitochondria and peroxisomes by binding specific adaptor proteins and lipids, both of which are integral to the limiting organellar membrane. In efforts to understand how such multivalent interactions regulate Drp1 functions, in vitro reconstitution schemes rely on recruiting soluble portions of the adaptors appended with genetically encoded polyhistidine tags onto membranes containing Ni²⁺-bound chelator lipids. These strategies are facile and circumvent the challenge in working with membrane proteins but assume that binding is specific to proteins carrying the polyhistidine tag. Here, we find using chelator lipids and chelator beads that both native and recombinant Drp1 directly bind Ni²⁺ ions. Metal binding, therefore, represents a potential strategy to deplete or purify Drp1 from native tissue lysates. Importantly, high concentrations of the metal in solution inhibit GTP hydrolysis and renders Drp1 inactive in membrane fission. Together, our results emphasize a metal-binding propensity, which could significantly impact Drp1 functions.

PI4P/PS countertransport by ORP10 at ER-endosome membrane contact sites regulates endosome fission

Membrane contact sites (MCSs) serve as a zone for nonvesicular lipid transport by oxysterol-binding protein (OSBP)-related proteins (ORPs). ORPs mediate lipid countertransport, in which two distinct lipids are transported counterdirectionally. How such lipid countertransport controls specific biological functions, however, remains elusive. We report that lipid countertransport by ORP10 at ER–endosome MCSs regulates retrograde membrane trafficking. ORP10, together with ORP9 and VAP, formed ER–endosome MCSs in a phosphatidylinositol 4-phosphate (PI4P)-dependent manner. ORP10 exhibited a lipid exchange activity toward its ligands, PI4P and phosphatidylserine (PS), between liposomes in vitro, and between the ER and endosomes in situ. Cell biological analysis demonstrated that ORP10 supplies a pool of PS from the ER, in exchange for PI4P, to endosomes where the PS-binding protein EHD1 is recruited to facilitate endosome fission. Our study highlights a novel lipid exchange at ER–endosome MCSs as a nonenzymatic PI4P-to-PS conversion mechanism that organizes membrane remodeling during retrograde membrane trafficking.

Endosomal recycling tubule scission and integrin recycling involve the membrane curvature-supporting protein LITAF

Recycling to the cell surface requires the scission of tubular membrane intermediates emanating from endosomes. Here, we identify the monotopic membrane protein LPS-induced TNF-activating factor (LITAF) and the related protein cell death involved p53 target 1 (CDIP1) as novel membrane curvature proteins that contribute to recycling tubule scission. Recombinant LITAF supports high membrane curvature, shown by its ability to reduce proteoliposome size. The membrane domains of LITAF and CDIP1 partition strongly into ∼50 nm diameter tubules labelled with the recycling markers Pacsin2, ARF6 and SNX1, and the recycling cargoes MHC class I and CD59. Partitioning of LITAF into tubules is impaired by mutations linked to Charcot Marie Tooth disease type 1C. Meanwhile, co-depletion of LITAF and CDIP1 results in the expansion of tubular recycling compartments and stabilised Rab11 tubules, pointing to a function for LITAF and CDIP1 in membrane scission. Consistent with this, co-depletion of LITAF and CDIP1 impairs integrin recycling and cell migration.

Defining the protein and lipid constituents of tubular recycling endosomes

Once internalized, receptors reach the sorting endosome and are either targeted for degradation or recycled to the plasma membrane, a process mediated at least in part by tubular recycling endosomes (TREs). TREs may be efficient for sorting owing to the ratio of large surface membrane area to luminal volume; following receptor segregation, TRE fission likely releases receptor-laden tubules and vesicles for recycling. Despite the importance of TRE networks for recycling, these unique structures remain poorly understood, and unresolved questions relate to their lipid and protein composition and biogenesis. Our previous studies have depicted the endocytic protein MICAL-L1 as an essential TRE constituent, and newer studies show a similar localization for the GTP-binding protein Rab10. We demonstrate that TREs are enriched in both phosphatidic acid (PA) and phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2), supporting the idea of MICAL-L1 recruitment by PA and Rab10 recruitment via PI(4,5)P2. Using siRNA knock-down, we demonstrate that Rab10-marked TREs remain prominent in cells upon MICAL-L1 or Syndapin2 depletion. However, depletion of Rab10 or its interaction partner, EHBP1, led to loss of MICAL-L1-marked TREs. We next used phospholipase D inhibitors to decrease PA synthesis, acutely disrupt TREs, and enable monitoring of TRE regeneration after inhibitor washout. Rab10 depletion prevented TRE regeneration, whereas MICAL-L1 knock-down did not. It is surprising that EHBP1 depletion did not affect TRE regeneration under these conditions. Overall, our study supports a primary role for Rab10 and the requirement for PA and PI(4,5)P2 in TRE biogenesis and regeneration, with Rab10 likely linking the sorting endosome to motor proteins and the microtubule network.

Eps15 Homology Domain Protein 4 (EHD4) is required for Eps15 Homology Domain Protein 1 (EHD1)-mediated endosomal recruitment and fission

Upon internalization, receptors are trafficked to sorting endosomes (SE) where they undergo sorting and are then packaged into budding vesicles that undergo fission and transport within the cell. Eps15 Homology Domain Protein 1 (EHD1), the best-characterized member of the Eps15 Homology Domain Protein (EHD) family, has been implicated in catalyzing the fission process that releases endosome-derived vesicles for recycling to the plasma membrane. Indeed, recent studies suggest that upon receptor-mediated internalization, EHD1 is recruited from the cytoplasm to endosomal membranes where it catalyzes vesicular fission. However, the mechanism by which this recruitment occurs remains unknown. Herein, we demonstrate that the EHD1 paralog, EHD4, is required for the recruitment of EHD1 to SE. We show that EHD4 preferentially dimerizes with EHD1, and knock-down of EHD4 expression by siRNA, shRNA or by CRISPR/Cas9 gene-editing leads to impaired EHD1 SE-recruitment and enlarged SE. Moreover, we demonstrate that at least 3 different asparagine-proline-phenylalanine (NPF) motif-containing EHD binding partners, Rabenosyn-5, Syndapin2 and MICAL-L1, are required for the recruitment of EHD1 to SE. Indeed, knock-down of any of these SE-localized EHD interaction partners leads to enlarged SE, presumably due to impaired endosomal fission. Overall, we identify a novel mechanistic role for EHD4 in recruitment of EHD1 to SE, thus positioning EHD4 as an essential component of the EHD1-fission machinery at SE.

Cellular functions and intrinsic attributes of the ATP-binding Eps15 homology domain-containing proteins

Several cellular processes rely on a cohort of dedicated proteins that manage tubulation, fission, and fusion of membranes. A notably large number of them belong to the dynamin superfamily of proteins. Among them is the evolutionarily conserved group of ATP-binding Eps15-homology domain-containing proteins (EHDs). In the two decades since their discovery, EHDs have been linked to a range of cellular processes that require remodeling or maintenance of specific membrane shapes such as during endocytic recycling, caveolar biogenesis, ciliogenesis, formation of T-tubules in skeletal muscles, and membrane resealing after rupture. Recent work has shed light on their structure and the unique attributes they possess in linking ATP hydrolysis to membrane remodeling. This review summarizes some of these recent developments and reconciles intrinsic protein functions to their cellular roles.

Sorting nexin 17 (SNX17) links endosomal sorting to Eps15 homology domain protein 1 (EHD1)-mediated fission machinery

Following endocytosis, receptors that are internalized to sorting endosomes are sorted to different pathways, in part by sorting nexin (SNX) proteins. Notably, SNX17 interacts with a multitude of receptors in a sequence-specific manner to regulate their recycling. However, the mechanisms by which SNX17-labeled vesicles that contain sorted receptors bud and undergo vesicular fission from the sorting endosomes remain elusive. Recent studies suggest that a dynamin-homolog, Eps15 homology domain protein 1, catalyzes fission and releases endosome-derived vesicles for recycling to the plasma membrane. However, the mechanism by which EHD1 is coupled to various receptors and regulates their recycling remains unknown. Here we sought to characterize the mechanism by which EHD1 couples with SNX17 to regulate recycling of SNX17-interacting receptors. We hypothesized that SNX17 couples receptors to the EHD1 fission machinery in mammalian cells. Coimmunoprecipitation experiments and in vitro assays provided evidence that EHD1 and SNX17 directly interact. We also found that inducing internalization of a SNX17 cargo receptor, low-density lipoprotein receptor-related protein 1 (LRP1), led to recruitment of cytoplasmic EHD1 to endosomal membranes. Moreover, surface rendering and quantification of overlap volumes indicated that SNX17 and EHD1 partially colocalize on endosomes and that this overlap further increases upon LRP1 internalization. Additionally, SNX17-containing endosomes were larger in EHD1-depleted cells than in WT cells, suggesting that EHD1 depletion impairs SNX17-mediated endosomal fission. Our findings help clarify our current understanding of endocytic trafficking, providing significant additional insight into the process of endosomal fission and connecting the sorting and fission machineries.